A graphene layer (a graphite sheet) is a monolayer of carbon atoms and a structure thereof shows a six-membered ring of carbon atoms that are densely packed. It has been known that electrons behave as relativistic particles having a mass of zero in the inside of the graphene layer to show a remarkably high electron mobility. Further, of the known substances, the graphene layer has the highest melting point and is also excellent in thermal conductivity. Accordingly, since the first report (see non-patent document 1), the graphene layer has become an active area of research among researchers. Under such situations, recently, various reports have been made on the graphene layer.
For example, non-patent document 2 reports that an electron mobility exceeding 200,000 cm2/Vsec has been measured in graphene. Such a high electron mobility corresponds to about 100 times the electron mobility of Si (silicon) and 5 times or more the electron mobility of carbon nanotubes. Further, non-patent document 3 reports that there is a bandgap of 0.26 eV between two-layer graphene and a SiC substrate, and proposes applications of graphene to transistors and other electronics.
As techniques for forming a graphene layer having the above-described excellent characteristics, for example, on a substrate, there have hitherto been widely known 1) a technique of transferring one or more graphene layers, which are exfoliated from graphite crystals showing a stacking structure (see patent document 1), onto a SiO2/Si substrate, 2) a technique of heating a SiC substrate under (ultra)high vacuum to sublime Si atoms and forming a graphene layer on the SiC substrate by self-organization of remaining C (carbon) atoms, and the like.
However, the technique of the above 1) has an inherent problem that when the graphene layer is exfoliated from the graphite crystals, it largely depends on chance, so that it is difficult to obtain a large-area graphene layer. Further, the graphene layer to be obtained according to the technique of the above 2) has relatively many irregularities in an atomic level, and the maximum length of the graphene layer is about 200 nm, which is not sufficiently large.
Accordingly, particularly in the semiconductor industry, development of a graphene/SiC composite material has been earnestly desired at present in which a large-area graphene layer that is flat in an atomic level is formed on a SiC substrate.